Transducer apparatuses of the type being discussed comprise a transducer housing having a cavity encased by a wall, typically a metal wall, as well as a tube having a lumen surrounded by a wall, typically likewise a metal wall, and arranged within the cavity of the transducer housing, in such a manner that, between an inner surface of the wall of the transducer housing facing the cavity and an outer surface of the wall of the tube, namely an outer, surface of the wall of the tube facing the cavity, an intermediate space is formed, most often an intermediate space filled with air or an inert gas. The at least one tube is, especially, adapted to guide in its lumen a fluid flowing, at least at times, for example, a fluid in the form of a gas, a liquid or a flowable dispersion, in such a manner that an inner surface of the wall of the tube facing the lumen is contacted by fluid guided in the lumen to form a first interface of a first type, namely an interface between a fluid and a solid phase. In order to measure a target temperature, namely a temperature of the respective transducer apparatus, equally as well a time variable temperature, at a predefined measuring, respectively reference, point within the respective transducer apparatus, such transducer apparatuses comprise, furthermore, at least two temperature sensors formed, in each case, by means of a temperature detector arranged within the intermediate space and, consequently, during operation not contacted by the fluid in the lumen of the at least one tube, wherein at least one of the temperature sensors has a coupling body connecting its temperature detector thermally conductively with the wall, for example, a coupling body formed by means of thermally conductive adhesive. Each of such temperature detectors can be, for example, a platinum measuring resistor, a thermistor or a thermocouple or, however, electrical circuits formed by means of a plurality of such temperature sensitive, electrical, respectively electronic, components. Each of the at least two temperature sensors is adapted to transduce a measurement location temperature corresponding to a temperature at a temperature measurement location formed by means of the respective temperature detector, in each case, into a corresponding temperature measurement signal, namely an electrical measurement signal representing the particular measurement location temperature, for example, an electrical measurement signal having an electrical signal voltage dependent on the measurement location temperature and/or an electrical signal current dependent on the measurement location temperature. Target temperature in the case of such transducer apparatuses can be, for example, a measured fluid temperature, namely a temperature of the fluid guided during operation of the transducer apparatus in the lumen of the at least one tube, and/or a tube temperature, namely a temperature of the wall of the tube contacted by the fluid respectively located in the lumen.
The transducer apparatus can, furthermore, be connected to a measuring and operating electronics, formed, for example, by means of at least one microprocessor, to form a measuring system for measuring at least one measured variable, for example, namely the temperature of the measured fluid or also a density and/or a viscosity of the fluid guided in the at least one tube of the respective transducer apparatus. The measuring and operating electronics can, in turn, be adapted, with application of the at least two temperature measurement signals generated by means of the transducer apparatus, to generate a measured value, which represents the at least one measured variable. In the case of such measuring systems, the measuring and operating electronics is typically accommodated within at least one, comparatively robust, especially impact-, pressure-, and/or weather resistant, electronics housing. The electronics housing can, for example, be arranged removed from the transducer system and connected with such only via a flexible cable; it can, however, also be directly arranged on the transducer housing, respectively affixed thereto. Further examples of transducer apparatuses of the type being discussed, respectively measuring systems formed therewith, are shown in, among others, European Application, EP-A 919 793, US-A 2004/0187599, US-A 2008/0127745, US-A 2011/0113896, U.S. Pat. Nos. 4,768,384, 5,602,346, 6,047,457, 7,040,179, 7,549,319, Published International Applications WO-A 01/02816, WO-A 2009/051588, WO-A 2009/134268, WO-A 2012/018323, WO-A 2012/033504, WO-A 2012/067608 or WO-A 2012/115639.
In the case of measuring systems of the above indicated type used in industrial measuring and automation technology, the particular measuring and operating electronics is usually electrically connected via corresponding electrical lines also to a superordinated electronic data processing system arranged most often spatially removed from the respective measuring system and most often also spatially distributed, to which the measured values produced by the respective measuring system and correspondingly carried by means of at least one of these measured value signals are forwarded near in time, for example, also in real time. Measuring systems of the type being discussed are additionally usually connected with one another and/or to corresponding electronic process controllers by means of a data transmission network provided within the superordinated data processing system, for example, to programmable logic controllers (PLC) installed on-site or to process control computers installed in a remote control room, where the measured values produced by means of the respective measuring system and digitized in suitable manner and correspondingly encoded are forwarded. By means of such process control computers, the transmitted measured values can be further processed and visualized as corresponding measurement results e.g. on monitors and/or converted into control signals for other field devices, such as e.g. magnet-operated valves, electric motors, etc., embodied as actuating devices. Since modern measuring arrangements can also be monitored and, in given cases, controlled and/or configured most often directly from such control computers, in corresponding manner, operating data intended for the measuring system are equally sent via the aforementioned data transmission networks, which are most often hybrid as regards the transmission physics and/or the transmission logic. Accordingly, the data processing system usually also serves to condition, for example, suitably to digitize and, in given cases, to convert into a corresponding telegram, the measured value signal delivered by the measuring system, corresponding to the requirements of downstream data transmission networks, and/or to evaluate such on-site. For such purpose, there are provided in such data processing systems, electrically coupled with the respective connecting lines, evaluating circuits, which pre- and/or further process as well as, in case required, suitably convert the measured values received by the respective measuring system. Serving for data transmission in such industrial data processing systems at least sectionally, especially serially, are fieldbusses, such as e.g. FOUNDATION FIELDBUS, RACKBUS-RS 485, PROFIBUS, etc., fieldbusses, or, for example, also networks based on the ETHERNET standards, as well as the corresponding, most often comprehensively standardized, transmission protocols. Alternatively or supplementally, in the case of modern measuring systems of the type being discussed, measured values can also be transmitted wirelessly per radio to the particular data processing system. Besides the evaluating circuits required for processing and converting the measured values delivered from the respectively connected measuring system, such superordinated data processing systems have most often also electrical supply circuits serving for supplying the connected measuring systems with electrical energy and providing a corresponding supply voltage, in given cases, fed directly by the connected fieldbus, for the respective electronics and the thereto connected electrical lines as well as for driving electrical currents flowing through the respective electronics. A supply circuit can, in such case, be associated with, for example, exactly one measuring system, respectively a corresponding electronics, and be accommodated together with the evaluating circuit associated with the respective measuring system, for example, combined in a corresponding fieldbus adapter, in a shared electronics housing, e.g. formed as a top hat rail module. It is, however, also quite usual to accommodate supply circuits and evaluating circuits in separate electronics housings, in given cases, spatially remotely from one another, and to connect them correspondingly together via external cables.
Transducer apparatuses of the type being discussed are applied not least of all also in vibronic measuring systems serving for ascertaining measured variables, for example, a mass flow rate, a density or a viscosity, of fluids guided in a process line, for example, a pipeline, respectively they can be an integral component of such a measuring system. Construction and operation of such vibronic measuring systems formed by means of such a transducer apparatus, for example, also measuring systems in the form of Coriolis, mass flow, measuring devices or also Coriolis, mass flow, measuring systems, are known, per se, to those skilled in the art and are described at length and in detail, for example, also in the above mentioned EP-A 919 793, US-A 2004/0187599, US-A 2008/0127745, US-A 2011/0113896, U.S. Pat. Nos. 4,768,384, 5,602,346, 7,040,179, 7,549,319, WO-A 01/02816, WO-A 2009/051588, WO-A 2009/134268, WO-A 2012/018323, WO-A 2012/033504, WO-A 2012/067608, WO-A 2012/115639, or, for example, also in US-A 2001/0037690, US-A 2011/0265580, US-A 2011/0146416, US-A 2011/0113896, US-A 2010/0242623, and Published International Applications, WO-A 2013/092104, WO-A 01/29519, WO-A 98/02725, WO-A 94/21999 or WO-A 88/02853. In the case of such vibronic measuring systems, the at least one tube of the respective transducer apparatus is, especially, also adapted, for the purpose of measuring the at least one measured variable, during operation, at least at times, to be caused to vibrate while filled with fluid to be measured, respectively flowed through by the fluid to be measured. Typically, the at least one tube is actively excited by means of at least one electromechanical oscillation exciter of the transducer apparatus acting thereon, for example, an oscillation exciter formed by means of a permanent magnet affixed to the at least one tube and by means of an exciter coil interacting therewith, to execute wanted oscillations, namely mechanical oscillations about a static resting position associated with the respective tube, especially also such mechanical oscillations, which are suitable to induce in the flowing fluid Coriolis forces dependent on a mass flow rate, m, and/or which are suitable to induce in the flowing fluid frictional forces dependent on a viscosity, η, and/or which are suitable to induce in the flowing fluid inertial forces dependent on a density, ρ. For registering mechanical oscillations of the at least one tube, not least of all also its wanted oscillations, the transducer apparatuses used in such vibronic measuring systems have, furthermore, in each case, at least one oscillation sensor, for example, an electrodynamic, oscillation sensor, which is adapted to produce at least one oscillatory signal, namely an electrical measurement signal representing oscillatory movements of the at least one tube, for example, with an electrical signal voltage dependent on a velocity of the oscillatory movements of the at least one tube. The measuring and operating electronics of such vibronic measuring systems is—not least of all for the case, in which the at least one measured value represents a density or a viscosity of the fluid guided in the at least one tube —, further adapted to generate the at least one measured value using both the at least two temperature measurement signals generated by means of the transducer apparatus as well as also the at least one oscillation signal, for example, in such a manner that the measuring and operating electronics ascertains the at least one measured value based on a wanted frequency measured based on the oscillation signal, namely an oscillation frequency of the wanted oscillations dependent on the measured variable to be measured and for this purpose metrologically compensates a possible dependence of the wanted frequency also on an instantaneous, measured fluid temperature, respectively a temperature distribution within the wall of the at least one tube.
In the case of modern measuring systems used in industrial measuring and automation technology, not least of all also in the case of vibronic measuring systems of the above indicated type, the measuring and operating electronics is most often formed by means of one or more microprocessors, in given cases, also implemented as digital signal processors (DSP), in such a manner that the measuring and operating electronics ascertains the respective measured values for the at least one measured variable by numerical processing of digital, sampled values of measurement signals of the respective transducer apparatus, for example, namely digital, sampled values won from the at least two temperature measurement signals, respectively the at least one oscillatory signal, and provided in the form of corresponding digital values. Besides the evaluation of the temperature measurement signals as well as the at least one oscillation signal, the measuring and operating electronics of vibronic measuring systems of the above indicated type serves typically also to generate at least one driver signal, for example, a harmonic and/or clocked, driver signal, for the at least one electromechanical oscillation exciter. The driver signal can be controlled, for example, as regards an electrical current level and/or a voltage level.
As evident, among others, from the above mentioned U.S. Pat. Nos. 4,768,384, 7,040,179, respectively US-A 2008/0127745, a special problem of ascertaining a temperature in transducer apparatuses of the type being discussed, be it a measured fluid temperature or a tube temperature, is that the measurement location temperatures registered by means of the at least two, in given cases, also three or more, temperature sensors correspond, first of all, in each case, actually only to a local temperature at exactly the temperature measurement location formed by means of the respective temperature detector, that, however, conversely, most often actually a local, respectively average temperature at another apparatus reference point, namely a reference point within the transducer apparatus remote from each of the temperature measurement locations, is of interest (target temperature), for example, namely—not least of all for the purpose of ascertaining the measured fluid temperature—a temperature within the lumen of the at least one tube, and/or—not least of all for the purpose of correction of a dependence of the wanted frequency on a spatial temperature distribution within the wall of the at least one tube—actually a spatially averaged tube temperature should serve as a target temperature. A further problem can additionally be that as a result of unavoidable time changes of the measured fluid temperature within the transducer apparatus regularly also dynamic heat equilibration processes can take place, which likewise, not least of all due to the only very limited number of temperature measurement locations, respectively due to their mutual spatial separation, can lead to defective measurement results in measuring systems formed by means of transducer apparatuses of the type being discussed, be it in the case of ascertaining the measured fluid temperature or, for instance, in the case of application of the transducer apparatus in a vibronic measuring system, in the case of which measured variables, such as e.g. the density and/or the viscosity of a fluid guided in the at least one tube or also a mass flow rate of a fluid flowing through the at least one tube, are ascertained based on wanted oscillations of the at least one tube. Moreover, such as also discussed, among others, in the above mentioned WO-A 2009/051588, also an ambient temperature of the transducer, namely a temperature of an atmosphere surrounding the transducer housing, respectively a time change of the ambient temperature, can degrade the accuracy, with which the measured fluid temperature, respectively the tube temperature, can be ascertained by means of such transducer apparatuses.
Further investigations on the part of the inventors have, furthermore, shown that, besides the above indicated influences, surprisingly, however, also a temperature difference, respectively its time change, existing between the measured fluid temperature and the tube ambient temperature, namely a temperature of the fluid volume in the intermediate space formed between the inner surface of the wall of the transducer housing and the outer surface of the wall of the tube, consequently the fluid volume surrounding the tube, can influence the respective temperature measurement signals. Fundamentally, namely each of the temperature sensors is via a respective surface facing the intermediate space thermally coupled—more or less strongly—also to the fluid volume kept in the intermediate space, in such a manner that a heat transfer taking place between the fluid within the lumen of the tube and the fluid volume surrounding the tube regularly leads partially also through the respective temperature sensors. Due to such a heat transfer, respectively, associated therewith, also due to heat transport processes respectively transpiring between each of the temperature sensors and the fluid volume formed in the intermediate space, the respective measurement location temperature is, thus, dependent not only on the tube —, respectively the measured fluid, temperature, but, instead, regularly also mentionably co-determined by the ambient temperature of the tube. Moreover, the inventors could also detect that the above-mentioned thermal coupling can, at times, assume such an extent that, as regards the high accuracy of measurement desired for measuring systems of the type being discussed, not least of all also for vibronic measuring systems, it is actually no longer negligible, respectively that, conversely, an ignoring of the influence of such temperature difference on the respectively registered measurement location temperature, respectively the temperature measurement signal representing such, can lead to quite significant measurement errors, for instance, in such a manner that the measured values for the target temperature ascertained, in each case, by means of the respective measuring system, especially also in the case of time constant target temperature, deviate, at times, by more than 0.5 K from the actual, respectively true, target temperature.